Conventionally, various ideas have been conceived for increasing the front wheel driving force of an asphalt finisher. A relatively simple method, shown in FIG. 9, is one in which, the axles of the front wheels are fixed to both sides of the body of the asphalt finisher, and the distribution of the load on the right and left front wheels while the asphalt finisher is operated on rough ground is ignored. In another method, a flow divider, which corresponds to a non-spin of differential gear or differential gear lock in a mechanically driven system is provided within the front wheel drive hydraulic circuit to prevent a loaded wheel from stopping or a wheel with no load from turning when the load on the wheels is deficient, so as to apply at least some tractive force to the loaded wheels. In addition, there is also a method in which, in order to achieve uniform right and left wheel loads, a swinging beam is disposed at the right and left lower portions of the front hopper so that tractive force can usually be generated by both wheels. In either case, it is necessary to set the driving force so that the driven wheel can maintain traction on the ground surface, assuming that there is no extra weight in the hopper. In the present state of the art, an all wheel drive asphalt finisher has its front and rear wheels driven by means of different oil hydraulic pumps and motors no matter what system is used for synchronizing the speeds of the front and rear wheels, and consequently, full synchronization is difficult to achieve. Therefore, in almost every case, the front wheel is usually rotated faster than the rear wheel so that the rear wheel is constantly pulled by the front wheel. In consequence, the wheels cannot attain maximum traction. The speed difference between the wheel and the ground surface causes slippage between the wheel and ground surface which reduces the tractive force and also damages the ground surface. In addition, although there is one method in which the driving force of the front wheel is controlled, this merely generates a front wheel driving force proportional to the hydraulic pressure force required to drive the rear wheel, which is generated by the entire running resistance (rolling resistance, damping resistance, screeding resistance and climbing resistance) of the asphalt finisher. Still further, in a finisher equipped with the above-described flow divider for preventing a wheel from turning when there is no load on it, the driving force is applied only to wheels which contact the ground surface when the asphalt finisher runs on rough ground, and it is not possible to generate a constantly stable tractive force. Further, even with a finisher equipped with the swinging beam, only a limited tractive force can be usually generated due to changing wheel load.
Further, with a finisher in which the front wheel driving force is controlled, its driving force can be controlled as the running resistance increases and decreases when a sufficient amount of weight is in the hopper, but it is still impossible to generate a driving force proportional to the gripping force of the tire. In addition, if there is not enough weight in the hopper, slippage of tires is likely to occur due to the tractive resistance.